Personally i do not belief that sotapanna magga stays unnoticed.
I also do not know any references for this belief in the Sutta-pitaka.
There are texts who describe that the Dhamma-eye arises or opens, or that there is some breaktrough to the Dhamma or Four Noble Truths. This is noticed.
I think that such a life-changing event can impossibly stay unnoticed.
If something has the impact to eradicate longstanding habits can it really stay unnoticed?
Anyway, if someone knows a reference for the view that sotapanna magga is not noticed i would appreciate it.